hello,
I use JK2radiant to make a map for JK2
I would like to insert a X-wing but i do not know how to process.
I load a prefab x_wing.map, in the entity i enter the key "model" and the value is the path "model/blabla/blabla.../blabla.md3"
When i load the map, i do not see the X-wing but it is here, it is invisible.

Erm, you just loaded up the physics clipping for an X-wing. For the model, you need a misc_model entity or similar. This kind of thing is covered in tutorials, and you should really avoid asking about stuff this simple, as it's already been mentioned before.

maybe your *show entities as*settings are that it shows it as a wireframe.
the rightmost button is a red block with a little dot, click it and set it to skinned. All models should be seen with textures than.